AudioPluginHost: Fix potential hang during plugin scanning

When dragging-and-dropping a plugin into the plugin host, it was
possible that the subprocess scanner could fail to send a response.
This would cause the superprocess to freeze, waiting for a reply.

With this change in place, the subprocess will always send a response
after scanning on the main thread.

extras/AudioPluginHost/Source/HostStartup.cpp

@@ -48,13 +48,15 @@ private:
         if (mb.isEmpty())
             return;
 
-        if (! doScan (mb))
-        {
-            {
-                const std::lock_guard<std::mutex> lock (mutex);
-                pendingBlocks.emplace (mb);
-            }
+        const std::lock_guard<std::mutex> lock (mutex);
 
+        if (const auto results = doScan (mb); ! results.isEmpty())
+        {
+            sendResults (results);
+        }
+        else
+        {
+            pendingBlocks.emplace (mb);
             triggerAsyncUpdate();
         }
     }
@@ -68,26 +70,17 @@ private:
     {
         for (;;)
         {
-            const auto block = [&]() -> MemoryBlock
-            {
-                const std::lock_guard<std::mutex> lock (mutex);
-
-                if (pendingBlocks.empty())
-                    return {};
+            const std::lock_guard<std::mutex> lock (mutex);
 
-                auto out = std::move (pendingBlocks.front());
-                pendingBlocks.pop();
-                return out;
-            }();
-
-            if (block.isEmpty())
+            if (pendingBlocks.empty())
                 return;
 
-            doScan (block);
+            sendResults (doScan (pendingBlocks.front()));
+            pendingBlocks.pop();
         }
     }
 
-    bool doScan (const MemoryBlock& block)
+    OwnedArray<PluginDescription> doScan (const MemoryBlock& block)
     {
         MemoryInputStream stream { block, false };
         const auto formatName = stream.readString();
@@ -106,20 +99,19 @@ private:
             return nullptr;
         }();
 
-        if (matchingFormat == nullptr
-            || (! MessageManager::getInstance()->isThisTheMessageThread()
-                && ! matchingFormat->requiresUnblockedMessageThreadDuringCreation (pd)))
+        OwnedArray<PluginDescription> results;
+
+        if (matchingFormat != nullptr
+            && (MessageManager::getInstance()->isThisTheMessageThread()
+                || matchingFormat->requiresUnblockedMessageThreadDuringCreation (pd)))
         {
-            return false;
+            matchingFormat->findAllTypesForFile (results, identifier);
         }
 
-        OwnedArray<PluginDescription> results;
-        matchingFormat->findAllTypesForFile (results, identifier);
-        sendPluginDescriptions (results);
-        return true;
+        return results;
     }
 
-    void sendPluginDescriptions (const OwnedArray<PluginDescription>& results)
+    void sendResults (const OwnedArray<PluginDescription>& results)
     {
         XmlElement xml ("LIST");
 
@@ -132,9 +124,6 @@ private:
 
     std::mutex mutex;
     std::queue<MemoryBlock> pendingBlocks;
-
-    // After construction, this will only be accessed by doScan so there's no need
-    // to worry about synchronisation.
     AudioPluginFormatManager formatManager;
 };